1. Field of the Invention
The invention relates to rotary drill bits for use in drilling holes in subsurface formations and, more particularly, to rotary drill bits having moveable formation-engaging members.
2. Background of the Related Art
Drill bits for use in drilling holes in subterranean formations include cutting structures that are positioned at selected locations on a bit body. Typically, each cutting structure includes a thin facing table of superhard material, such as polycrystalline diamond, that is bonded to a substrate of a softer material, such as tungsten carbide. The general construction of bits of this kind is well known and will not be described in detail.
During drilling operations, the cutting structures of such a bit may be subject to impact loads which may cause the cutting elements to crack or fracture. Such impact loads may be generated, for example, when tripping the drill bit into or out of the borehole, or when raising or lowering the drill bit temporarily at the bottom of the borehole. Also, such impact loads may occur when the drill passes through a comparatively soft formation and strikes a significantly harder formation, or when the drill bit encounters hard occlusions within a generally soft formation.
In addition, such drill bits may be subject to instability and vibration. Also such drill bits may be subject to the phenomenon known as "bit whirl," where the bit tends to precess around the borehole in the opposite direction to the direction of rotation of the bit about its axis. Bit whirl may lead to the drilling of an oversize borehole, as well as other difficulties. For example, bit whirl may result in cutting structure momentarily moving in the reverse direction relative to the formation, which can lead to the chipping of the diamond layer on the cutting element. In extreme cases, bit whirl may lead to breakage of all or part of the diamond layer away from its substrate, or even to the separation of the cutting element as a whole from the stud on which it is mounted.
The present invention may address one or more of the problems set forth above.